1. Field of the Invention
The present invention relates to alignment of optical components, and more particularly, to alignment of reflective and refractive optical components in high precision optical systems.
2. Related Art
Most multiple lens assemblies are currently aligned using one (or more) of the following methods:
(a) Mechanical indicators are used for either (or both) centering the outside diameter and minimizing the apparent wedge between lens surfaces relative to the lens cell;
(b) Alignment telescopes can be used for aligning centers of curvatures of the lens elements to a common optical axis;
(c) Fabricating the lens elements and the lens cell to very tight optical and mechanical tolerances, so that a “slip fit” of the elements in the cell results in an aligned system; and
(d) Coarsely assembling the lens, measuring the lens' wavefront and distortion across its field of view, and calculating the adjustments required to each lens element to minimize the wavefront error and distortion.
For optical systems requiring diffraction-limited performance (as needed for lithography optics), the first three of these techniques do not have the necessary alignment accuracy. To even get close to diffraction-limited performance, state-of-the-art mechanical and optical measuring systems are required. Optimizing the alignment using measured wavefront and distortion data requires either of the first two alignment methods to be performed as a starting point. The alignment process that uses the measured wavefront and distortion data is an iterative process. Because of cross-coupling of errors in the optical system, several measurements and alignment adjustments are required to successfully align a system. The exact number of iterations required to align a system depends on the designed quality.
Aligning an optical system using mechanical indicators does not account for homogeneity errors that can have the same effect as a mechanical wedge. Mechanical indicators and their related tooling (air bearing rotary tables, etc.) do not have the required accurately to align high quality optical systems, such as lithography optical systems. Because a mechanical probe or an air gauge must either be in contact, or be in very close proximity, to the lens element being aligned, there are frequently mechanical interferences with the lens cell structure. The probe is actually measuring an extremely small region on the lens surface. This region may not accurately represent the full optical surface.
An alignment telescope's sensitivity is limited by the angular resolution of its optical system, the distance between the lens being aligned and the alignment telescope, and how well the alignment telescope optics are aligned. Commercially available alignment telescopes do not have the required accuracy. A custom-designed and fabricated alignment telescope has a limited range over which it can be used, because it works only for a limited range of lens radii of curvatures. This results in the need to build at least several alignment telescopes (or additional optical elements and mechanical components to an existing alignment telescope), each of which has to be aligned to tolerances close to what is required for a lithography lens. Alignment telescopes are difficult to use on short radii of curvature lens surfaces, due to the small amount of light captured by the alignment telescope aperture. Alignment telescopes are also not usable with lenses and mirrors that have aspheric surfaces. The asphericity causes the image reflected off the surface being aligned to be badly aberrated, making it impossible to achieve fine alignment tolerances.
Measuring an optical systems wavefront and distortion, and then back-calculating the alignment errors, is very time consuming and difficult, unless one starts with the optical system being relatively close to the optimum alignment condition. Multiple alignment iterations are required because of the cross coupling of the alignment aberrations between all the surfaces.
Accordingly, there is a need in the art for a fast and simple method of aligning optical surfaces.